A variety of devices have been disclosed in the medical field for slowly and continuously infusing drug solutions such as heparin, antibiotics, and chemotherapy agents. In one such device, a drug is contained in an elastic balloon reservoir that gradually deflates to deliver the medication through an orifice of selected size. Another device disclosed in U.S. Pat. No. 4,561,856 utilizes a gas spring to drive a syringe piston and cause liquid to be discharged from a syringe section. U.S. Pat. No. 4,180,067 describes a vacuum cylinder and a flow-restricting valve as the driving means for forcing fluid from a container at a controlled rate. Other patents, such as U.S. Pat. No. 4,265,241, show devices in which a pressure differential is created by pressurizing one chamber and then allowing the gas to pass into another chamber, equalizing pressures and shifting a piston as such equalization occurs (FIG. 5). U.S. Pat. Nos. 4,335,835, 4,360,019, 2,547,099, and 2,865,371 disclose variations of pressure delivery systems. U.S. Pat. No. 4,505,710 (FIG. 1) shows a membrane that serves as a one-way metering valve through which liquid flows under pressure into a receiving chamber. As the receiving chamber expands, a drug in an adjacent chamber is slowly expelled for delivery to a patient.
All of such prior devices are believed to have major shortcomings or disadvantages. In some cases, flow rate is determined by the passage of a drug through an orifice and, therefore, flow rate is dependent on the particular viscosity of the drug to be delivered. Other devices are relatively difficult to fill, or are designed so that flow rate cannot be readily adjusted by a physician, or utilize membranes and other components that may not be compatible with certain types of drugs, or are relatively complex and expensive in construction, or, in the case of operative failure, tend to deliver a drug at an increased (rather than decreased) dosage rate.
The present invention is directed to a relatively simple and reliable device for delivering a drug at a rate that may be readily controlled by a physician and is independent of the viscosity of the particular drug to be infused. The device is easily filled (in essentially the same manner as a standard syringe), has no greater problems of drug compatibility than standard syringes, and may be filled either during manufacture or at the time of use. The power module uses a vacuum chamber and, in the event of accidental or unintended vacuum loss, the rate of drug delivered to the patient decreases rather than increases. In a preferred embodiment, vacuum loss is also visually and easily detectable by means of a vacuum loss indicator.
Briefly, the device comprises a syringe section and a power section. The power section includes a casing having a first chamber containing a viscous liquid and a second chamber under partial vacuum. A piston is slidably received in the casing and constitutes a movable wall of the first chamber. That piston is connected to the syringe section in such a way that retraction of the piston causes displacement of the syringe plunger. In use of the device, a metering passage between the two chambers is opened to allow the viscous fluid to flow into the evacuated second chamber. As the volume of viscous liquid in the first chamber is reduced, the piston of the power section retracts to drive the plunger of the syringe section forwardly, thereby dispensing the drug from the syringe at a predetermined rate.
In a preferred embodiment, the two sections comprise separate modules that are either connected together at the time of manufacture or immediately prior to use. The latter is feasible because, among other things, operation of the power module is not dependent on drug viscosity and, therefore, the same power module might be connected to any of a variety of syringe modules containing different drugs. Once connected, the two modules are constructed to prevent or resist separation, thereby precluding reuse of the parts. Ideally, during the initial procedure of connecting the two modules together, a bolus of drug is expelled from the syringe to clear the administration line of air. Coupling of the modules thereby arms the device for immediate use despite the fact that subsequent administration of drug will occur at a reduced rate of flow. The particular rate of flow of the viscous fluid through the metering passage, and hence the rate of administration of the drug delivered by the syringe unit, may either be preset during manufacture or may be set at the time of use by the pharmacist or physician. In the latter case, such adjustment is achieved simply by rotating a valve member provided by the power module into any selected position of adjustment indicated by a delivery-rate scale on that module. Should a later change in delivery rate, or an interruption in drug delivery, be deemed desirable by the physician, such an adjustment may be made by resetting the knob of the valve unit.
While differences in drug viscosity do not significantly alter the delivery rate of the device, substantial changes in the temperature of the viscous fluid of the power unit could have such an effect if it were not for a self-regulating feature incorporated into the system. Where the device is worn continuously in contact with the body the operating temperature would be relatively constant and no such self-regulation is necessary. The self-regulating feature is advantageous where, for example, a patient may wish to detach the device from his (her) body while resting in bed and, under such conditions, the temperature of the device would be significantly lowered. By including in the vacuum chamber a small amount of water or other volatile fluid that is immiscible with the viscous medium, temperature-dependent changes in viscosity of the viscous fluid medium may be substantially offset by changes in the vapor pressure of the regulating fluid, with the result that generally uniform drug delivery rates may be achieved even at different operating temperatures.
Other features, advantages and objects will become apparent from the specification and drawings.